Plasma generator

ABSTRACT

Plasma generator which is constructed and operated to provide an enhanced probability of collisions between charged and neutral particles in the working chamber together with enhanced energy transfer and uniformity of the plasma. The plasma generator includes a chamber (1) with means to produce electrons (5) and to cause the electrons to rotate and spiral (6,7) to produce ion of gases introduced into the chamber to produce a plasma. The plasma is contained by magnetic mirrors (10,11) at each end of the chamber (2). Axial oscillation of the plasma is produced by a low frequency oscillating potential (9) in the chamber to significantly increased ion electron interaction.

This invention relates to a technique which is used to expand andintensify a plasma from a source region into a working chamber.

In the last two decades, but particularly in recent years, significantdevelopments have taken place in the area of plasma generation. Thesehave been prompted by a usefulness of plasmas in all aspects ofsemiconductor technology and by an ever increasing number of newapplications. Some of the areas where a plasma, or its separated chargedparticles are used are ion sources, ion rockets, nuclear physics,heavy-ion science, ion plating, crystal growth (ion beam epitaxy),synthesis of compound materials (plasma polymerization, reactivesputtering), ion sputtering activated reactive evaporation, surfaceanalysis, medical applications, surface treatment, ion-assisted thinfilm deposition, lasers and many others.

As an example of the art reference may be had to the Proceedings of theInternational Engineering Congress--ISIAT'83 and 1PAT'83 Kyoto (1983) inwhich a plasma system is described which is used for plasma oxidation ofsilicon surfaces as used in VLSI production, but the present inventionhas many applications.

Also reference may be had to the specification of U.S. Pat. No.3,660,715 of Richard F. Post assigned to the United States EnergyCommission which relates to a plasma generator using a stack of pulsedwashers to release, ionize and heat the gas.

An object of the present invention is to provide a plasma generatingdevice of simple construction and ease of operation and which allows anenhanced collision probability between charged and neutral particles inthe working chamber together with enhanced energy transfer anduniformity of the plasma.

The invention consists of a plasma generator which allows both electronsand ions to oscillate in an applied field at low frequency excitationwith electrons and ions moving in opposite directions.

According to this invention a plain cylindrical magnetron communicateswith a chamber and both are pumped through by a high vacuum pumpingsystem, the magnetron having means to produce electrons and includingmagnetic means to cause the electrons to rotate and spiral and ionisegas atoms or molecules introduced to the magnetron to produce plasma,characterised by means to establish an axial oscillation of electronsand ions in opposite direction, the means comprising magnetic mirrormeans at the outlet of the magnetron adjacent to the chamber and furthermagnetic mirror means at the opposite side of the chamber whereby toincrease significantly ion electron interaction to facilitate multipleionization and additionally to enhancement of neutral particleionization, the chamber having in it an electrode adjacent to the plasmafield which is polarised to produce either an electrically neutral orpositive or negative stream of charged particles.

To enable the invention to be fully understood, it will now be describedwith reference to the accompanying drawings which show various forms ofthe invention and in which:

FIG. 1 is a schematic diagramatic view of one form of the inventionusing three magnets with one magnet related particularly with themagnetron and two magnets positioned one each side of the chamber toform the magnetic mirror means across the chamber, the drawing includingblock diagrams to show the method of establishing the axial of electronsand ions in opposite direction,

FIG. 2 is a somewhat schematic transverse section of the invention,

FIG. 3 is a view corresponding to FIG. 1 but showing a two magnetsystem, and

FIG. 4 shows in a view similar to FIG. 1 in which a single magnet isused.

Referring first to FIGS. 1 and 2, the two main components of the sourceare a plain cylindrical magnetron 1 and a vacuum chamber 2. The vacuumchamber 2 and the magnetron 1 are pumped through the opening 3 by aconventional high vacuum pumping system.

The materials to be ionized are introduced into the system through inlet4 in a gas or vapour form.

The initial ionization takes place in the plain cylindrical magnetron 1,which has an electron source 5, provided by a heated tungsten ortantalum or other filament placed at or near the magnetron axis, acylindrical anode 6 and an axial magnet 7 forming a magnetic fieldElectrons emitted from the filament are confined radially and preventedby the magnetic field from reaching the anode 6. The rotating andspiralling energetic electrons ionise gas atoms or molecules present inthe magnetron 1, forming a confined plasma 8, which persist as long assuitable conditions are maintained.

According to this invention, the intensity of the plasma is increased byestablishing an axial oscillation of electrons and ions. This may beachieved if consideration is given to the rate at which ions may respondto axial forces. Generally, with respect to electrons in a plasma, ionsare considered stationary or of low mobility due to their very muchlarger mass compared to electrons. However, we have found that if asuitable low frequency potential is applied along the magnetron axis,both electrons and positive ions can be made to oscillate axially.Negative ions, which are the result of electron attachment, also move inopposite direction to the movement of the positive ions so that theseare also subjected to collision with the positive ions. Ions achieve nonett movement if a high frequency potential is applied.

The nature of this mass transport is such that particles with oppositecharge polarity will move in opposite directions under the influence ofthe applied potential and this transportation mode will increasesignificantly the probability of ion-electron and ion-ion interaction,facilitating ionised molecule fracture and multiple ionization inaddition to an enhancement of neutral particle ionization.

The frequency used may depend on the nature of the ions but with gasions produced by admitting Hydrogen, Argon, Nitrogen, Methane or othersimilar gases or vapours to the magnetron, it has been found that afrequency of oscillation of 50 Hz is effective, but the frequency can beselected over a wide range. Beyond 1 MHz ions are unaffected by theapplied field.

To facilitate the energy transfer described above the magnetron 1 vacuumchamber 2 combination is used as shown in FIG. 2, where the lowfrequency voltage is applied between the magnetron 1 and the vacuumchamber 2 by the AC power supply 9 as indicated in FIG. 1. To containthe plasma and also to enhance further the process of ionization amagnetic field in the form of a magnetic mirror is formed by the fieldof magnet 10 and 11 as shown in FIGS. 1 and 2. The magnet 7 of themagnetron also forms a magnetic mirror with magnet 11.

While the magnetic mirrors have little or no effect on the ions theylargely control electron trajectories under static conditions. However,when the axial potential variation is applied above a certain voltagevalue, the electrons will move in an axial direction with sufficientenergy to ionize additional gas particles. They will alternately movebetween the magnetron 1 and the vacuum chamber 2 as driven by the lowfrequency voltage gradient of the AC power supply 9. Similarly thepositive ions are made to move by the same potential variation in theopposite direction to that of electrons or negative ions.

As was mentioned earlier the result of interaction of the chargedparticles with each other or with neutral atoms or molecules generatesmore ionised particles, which will also be influenced by the lowfrequency axial potential.

The chamber 2 has in it electrodes 12 and 13.

In the arrangement shown, the vacuum chamber 2 is at earth potential andthe magnetron chamber wall is connected through the AC power supply 9 tohave the necessary low frequency applied thereto, a DC power supply 14supplying the current for the filament 5 through the DC filament supplyunit 15.

A DC power supply 16, acting through an AC power supply 17, energisesthe electrode 12, these units be such as to allow both voltage andfrequency selection at the electrode 12 for relative deposition.

In FIG. 3 the magnet 7 of the magnetron extends to terminate adjacent tothe chamber 2 so that the magnetron magnet is common to the chamber.

In this figure is shown optionally how the electrical coupling betweenthis magnetron and the chamber 2 can be increased by providing anintermediate volume 18 for plasma extension.

In FIG. 4 a single magnet 19 is used having one pole 20 adjacent theouter end of the magnetron and its other pole 21 adjacent to the side ofthe chamber 2 remote from the magnetron.

In FIGS. 3 and 4 similar components are similarly numbered.

The electrodes 12 and 13 may support substrates for there filmdeposition from ionic state under suitable bias potential conditions.

When a series of DC and AC voltage combinations is used for theextraction of ionized particles, the phase of the AC extractionpotential must be out of phase of the axial low frequency potential by180° and the same frequency potential should be used.

While the plasma in the chamber can be maintained by using a suitable DCvoltage between the magnetron and the chamber, the plasma tends tospread into the gas supply line, but this does not happen with ACexcitation.

It is found that AC excitation together with the DC plus AC extractionprovides a simple way to overcome possible surface and space chargeaccumulation on and near substrates exposed to the electrically chargedparticle stream.

It is possible, as shown in FIG. 3, but applicable to each embodiment,to use a suitable cross magnetic mirror field as generated between thetwo magnets 22 and 23, or a single magnet as used in FIG. 4 couldprovide a transverse field, to further enhance the plasma generation.

Features of this plasma generator are:

(1) Its simplicity of structure, easy operation and uniform plasmaexcitation at pressures in the 10-4 Torr range.

(2) The low frequency excitation allows not only the electrons, but alsothe ions to oscillate at the applied field frequency, increasing theprobability of collision between charged particles and neutrals, thusincreasing the energy transfer to the plasma and the uniformity of theplasma.

(3) The plasma confinement as arranged reduces loss of the plasma, atthe same time allows easy access for utilization of the plasma.

(4) There are a number of ways to achieve interaction between probes andelectrodes and the plasma, of which two examples are given by theelectrode 12 and the electrode 13. Electrode 12 can be extended to forma continuous cylinder or a larger number of electrodes or extractors.

(5) The extraction of ionized particles is achieved by a series of DCand AC voltage combination, applied to an electrode such as 12 as shownparticulatly in FIG. 2, that can provide either an electrically neutral,positive or negative stream of particles as desired.

What is claimed:
 1. A plasma generator adapted to be evacuated, andopening to a chamber (2) a magnetron (1) having a cylindrical anode (6)and an axial magnet (7) to form a confined plasma (8) said chamber (2)including an electrode (12-13) at the plasma field, characterised bymagnetic means (10-11) effective at each side of the said chamber (2)and coaxial with the cylindrical anode (6) and polarised to form amagnetic mirror field, further characterised by means (9) to apply anaxial potential to the electrons and ions of the plasma (8) of afrequency low enough to establish an axial oscillation of the heavierions with electrons and ions moving in opposite directions in themagnetic mirror field, whereby to facilitate multiple ionization andenhancement of neutral particle ionization, further characterised bymeans (16-17) to polarize said electrode (12-13) to produce either anelectrically neutral or positive or negative beam of charged particles.2. A plasma generator according to claim 1 wherein the said lowfrequency axial potential is applied between the said anode (6) and thesaid chamber (2).
 3. A plasma generator according to claim 1 wherein thesaid magnetron (1) receives its electron supply from a filament (5)connected to a DC power source (15) which is biased by a further DCsource (14) connecting it to the said anode (6) of the magnetron (1). 4.A plasma generator according to claim 1 wherein the said electrode(12-13) at the plasma field is energised by an AC power source (17)biased by a DC power source (16) connected between the said AC powersource (17) and the said chamber (2).
 5. A plasma generator according toclaim 1 wherein the magnetic mirror at the magnetron (1) side of thesaid chamber (2) is common to the magnetron (1).
 6. A plasma generatoraccording to claim 1 wherein a single magnet (19) is used having onepole (20) adjacent to the end of the magnetron (1) remote from thechamber (2) and the other pole (22) adjacent the chamber (2) remote fromthe magnetron (1).
 7. A plasma generator according to claim 1 wherein amagnetic mirror field is applied through the chamber (2) at right anglesto the axis of the plasma generator (1) and at the centre of the firstmagnetic mirror field by magnet means (22-23).
 8. A plasma generatoraccording to claim 1 wherein the chamber (2) includes an intermediatevolume (18).
 9. The method of producing a plasma which consists inexciting a gas in a magnetron directed into an evacuated chamber,forming a magnetic mirror by means of opposite polarity magnetic fieldsdisposed one on one side of the said chamber and the other on the otherside of the said chamber about the axis of the said magnetron, applyingan oscillating field between the anode of said magnetron and saidchamber of a frequency low enough to oscillate both ions and electronsand of a voltage high enough to drive ions and electrons through themagnetic field between the said magnetron and the said chamber, andapplying a biased field to an electrode in said chamber whereby toproduce either an electrically neutral or positive or negative beam ofcharged particles.
 10. The method of producing a plasma according toclaim 9 which includes applying a further magnetic mirror field throughthe chamber at right angles and central to the magnetic mirror fieldexisting in the chamber, to further enhancing plasma generation.